Operate and maintain (organizational and Intermediate level) marine boilers, pumps, forced draft blowers, and heat exchangers; perform tests, transfers, and inventory of lubricating oils, fuels, and water.

This job was previously performed by Boiler Technicians, however, the BT rating merged with Machinist's Mate in 1997.

The other major place to find Machinist's Mates is in the auxiliaries division of engineering department, often referred to as "A-gang".

To include the operation and preventive maintenance of: air conditioning and refrigeration units, liquid oxygen and nitrogen units, hydraulic lifting and hoisting gear, hydraulic power plant components and mechanisms, chilled and fresh water, atmosphere control, ventilation, emergency diesel and corresponding sub-systems, ballast control, auxiliary drain, compressed air and gases, plumbing, trash disposal & other equipment as designated by the Engineering Officer (Chief Engineer, or CHENG).

MMNs earn a N25O Navy Enlisted Classification (NEC) (surface) or N15O (submarines) upon completion of prototype.

Along with the normal duties of a propulsion mechanic, they are responsible for the operation, maintenance, and repair of the auxiliary equipment in support of the Naval reactor(s) under their care.

The MMN must wait 5 years and be an E-5/E-6 to earn a N25S (surface both CVN Class) or N15S (submarines) Navy Enlisted Classification.

ELTs are water chemistry and radiological controls experts for their commands in addition to maintaining other MMN watchstation qualifications.

They go through a two-month school learning welding, brazing, and soldering from Hull Maintenance Technician senior enlisted welders.

The range of machinist's mate ratings include:[2] OPNAV INSTRUCTION 1220.1D CHANGING OR REMOVING PRIMARY NAVY ENLISTED CLASSIFICATION CODES FOR NUCLEAR PROPULSION PLANT OPERATORS.